1. Field of the Invention
The present invention relates to a toner for electrophotography and a developer composition containing the toner. More specifically, the present invention relates to a toner for electrophotography having excellent transparency and excellent surface smoothness of a fixed image demanded particularly for a color toner, and a developer composition containing such a toner.
Further, the present invention relates to a nonmagnetic toner for electrophotography used in a non-contact heat-fixing method and a developer composition containing such a toner.
2. Discussion of the Related Art
As disclosed in U.S. Pat. Nos. 2,221,776, 2,297,691, and 2,357,809 and other publications, conventional electrophotography comprises the steps of evenly charging a photoconductive insulating layer (a charging process); subsequently exposing the layer to eliminate the charge on the exposed portion, to thereby form an electrostatic latent image (an exposing process); visualizing the formed image by adhering colored charged fine powder, known as a toner, to the latent image (a developing process); transferring the obtained visible image to an image-receiving sheet such as a transfer paper (a transfer process); and permanently fixing the transferred image by heating, pressure application or other appropriate means of fixing (a fixing process).
Developing methods suitably used for the above electrophotographic methods can be roughly classified into dry-type developing methods and wet-type developing methods. The dry-type developing methods may be further classified into a developing method using a one-component developer and a developing method using a two-component developer, depending upon whether or not a carrier is used in the developer composition.
A color electrophotography using a color developer can be basically carried out by repeating the above processes of charging, exposing, developing, and transferring for a plural number of times, and subsequently fixing the transferred image, to thereby give a color fixed image.
When using a two-component dry-type developer, in the above developing process, toners are generally blended with carrier particles such as iron powders in a developing unit to thereby generate electrostatic charges in the toners by the frictional forces. At this time, brittle toners, namely toners having small pulverization indices, are likely to break upon the friction with the carrier particles when used for a long period of time, to thereby form fine particle toners, and the fine particle toners are likely to be adhered to the carrier surface. Further, since the fine particle toners are melt-fused with the carrier particles, the charge-supplying function of the carrier is lowered, which in turn results in the lowering of the triboelectric charges of the toners. As a result, the poorly charged toners are likely to be scattered, thereby causing background in the formed images.
On the other hand, as for binder resins for toner used for copy machines utilizing conventional non-contact heat-fixing methods such as an oven fixing method and a flash fusing method, resins which quickly fuse upon application of heat, etc. are required, so that a low-molecular weight binder resin is suitably used. However, since a low-molecular weight binder resin is likely to produce a brittle toner, similar problems to those mentioned above are likely to take place. Therefore, styrene-acrylic resins presently used as a main binder resin for electrophotography share these problems.
Particularly in the case where a nonmagnetic one-component developer is used, a thin toner layer is usually formed on a developer sleeve by a thin layer-forming blade. However, when a brittle toner is used, toner undesirably is fused on the developer sleeve upon the thin layer formation, so that the image quality of the formed images is lowered.
From the above, a development of a binder resin for toners which has excellent impact resistance and gives high quality image even in the long-term use is in demand.
On the other hand, as the copying machines are more generalized, they are highly likely to be used under severe environmental conditions, for instance, under high-temperature, high-humidity conditions or under low-temperature, low-humidity conditions. Therefore, a toner capable of obtaining formed images as clear as those obtained under normal conditions even under such severe environmental conditions is becoming increasingly crucial.
There are many toners where the electric properties largely varied in triboelectric charges and insulation resistance under high-temperature, high-humidity conditions or low-temperature, low-humidity conditions, thereby resulting in the deterioration of the formed images, even though the electric properties are in preferred ranges for the normal environmental conditions. For instance, under the low-temperature, low-humidity conditions, the image density of the formed images is lowered, and under the high-temperature, high-humidity conditions, the image density of the formed images becomes high. In the case where the triboelectric charges are remarkably lowered, most of the solid image portions contain color-eliminated inner portion. Particularly in the case of a polyester having at each end hydrophilic carboxyl groups and hydroxyl groups, when the concentration of the groups is high, namely an acid value thereof and a hydroxyl value thereof are high, the resulting toner made of the polyester is easily affected by the environmental conditions.
Therefore, a toner which is capable of forming clear fixed images in all environmental conditions is in demand, to thereby form fixed images no different from those obtained under normal environmental conditions.
Also, in a full-color electrophotography, by carrying out the developing method for a number of times, various toner layers with different colors are laminated on the same image-receiving member. In the color electrophotography, the following criteria are required for binder resins for toners.
(1) The fixable toner obtained therefrom has to be in a completely melting state completely losing the original shape of the toner particles so as not to inhibit the color reproducibility by optically diffused reflection. PA0 (2) Binder resin has to have a good transparency so as not to inhibit the toning of the under toner layers having different colors among the laminated toner layers. PA0 (1) A toner for electrophotography comprising at least a binder resin and a colorant, the binder resin containing a linear polyester as a main component and having a pulverization index of from 14 to 40; PA0 (2) The toner for electrophotography described in (1) above, wherein the linear polyester is obtainable by a condensation polymerization between an alcohol component and an acid component containing an aliphatic dicarboxylic acid in an amount of not less than 40 mol % of the entire acid component; PA0 (3) The toner for electrophotography described in (1) or (2) above, wherein the linear polyester has an acid value of not more than 40 KOH mg/g and a hydroxyl value of not more than 40 KOH mg/g; PA0 (4) The toner for electrophotography described in any one of (1) to (3) above, wherein the linear polyester has a softening temperature determined by a koka-type flow tester of from 80.degree. C. to 120.degree. C., a temperature difference between a flow beginning temperature and the softening temperature being from 10.degree. to 40.degree. C.; PA0 (5) The toner for electrophotography described in any one of (1) to (4) above, wherein the toner for electrophotography is a color toner for electrophotography; and PA0 (6) A developer composition comprising a magnetic carrier and the toner for electrophotography described in any one of (1) to (5) above.
As mentioned above, the binder resins for toners used in full-color copy machines not only have to have a wide fixing temperature region, but also good transparency and flatness of the fixed image surface upon fixing. Therefore, in addition to the properties required for binder resins for toners used in mono-color copy machines, such as a wide fixing temperature region and high offset resistance, more criteria have been required.
Therefore, an improved method in offset resistance for a mono-color toner cannot simply be applied for a binder resin for a full-colored toner. For instance, methods of improving offset resistance by forming a three-dimensional structure in the polyester by using polycarboxylic acids are disclosed in Japanese Patent Laid-Open No. 57-109825 and Japanese Patent Examined Publication No. 59-11902. However, in these methods, although the offset resistance can be improved, since the amount of acid components for crosslinking becomes large, the resulting toner has a large elasticity, so that the fixed image surface would not become flat in a relatively low temperature region, thereby causing problems in color reproducibility when used as a full-colored toner.
As explained above, it has been conventionally extremely difficult to satisfy the properties of having a suitable hardness, having basic toner characteristics, such as a triboelectric stability and a low-temperature fixing ability, and also having full-colored toner characteristics, such as transparency and smoothness of the fixed image surface, all at the same time.